Highly active antiretroviral therapy (HAART) has been shown to significantly lower morbidity and mortality rates from HIV-l infection by reducing levels of plasma virus to below detectable limits in many instances. However, recent studies indicate that there remains a small population of resting CD4+ T cells containing inducible, replication competent provirus in patients receiving HAART for extended periods of time. This latent HIV-1 reservoir creates an obstacle for the successful eradication of the virus. Previous observations suggest that in latently infected cells HIV transcription is blocked mainly at the elongation step, which is regulated positively and negatively by host cellular transcription factors and the viral protein Tat. We have demonstrated that a cellular protein complex, negative elongation factor (NELF) is recruited to the HIV-1 promoter in a virus specific manner. The RD subunit of this complex binds directly to HIV-l TAR in the presence and absence of Tat and the positive elongation factor 13(P-TEF[3), and phosphorylation of RD by the kinase activity of P-TEFI 3 abolishes this interaction with TAR. The block to productive HIV-1 transcription caused by the binding of NELF to TAR may indicate a reason for the detection of predominantly short, promoter proximal transcripts by various assays observed in the latently infected PBMCs derived from HAART responders. Therefore NELF may be one of the key players involved in the transition from nonproductive to productive HIV-1 transcription. We hypothesize that NELF is a factor contributing to the low level of HIV transcription in latently infected cells. In this proposed study, we will define the precise roles of NELF in Tat-dependent and Tat-independent transcription using a newly developed siRNA technology and a highly sensitive assay in a model cell line, as wells as PBMCs collected from HIV-1 infected patients. Moreover, we will attempt to activate viral transcription in latently infected cells by regulating NELF activity in hopes of developing potential treatments to reduce or abolish this latent reservoir. The results obtained from this study will increase our understanding of HW-1 pathogenesis and may suggest new therapeutic directions. [unreadable] [unreadable]